Marek's disease is a naturally occurring lymphoproliferative disease of chickens which is caused by a herpesvirus (Marek's disease virus, MDV). To determine whether the cells in Marek's disease lymphomas are homogeneous or consist of several lymphocytic subpopulations, lymphoid cells from a MDV infected group of chickens were compared with lymphoid cells from vaccinated chickens and uninfected chickens using computer assisted microphotometric analysis (CAMA). CAMA is a quantitative technique capable of differentiating cell populations based upon slight differences in the chromatin and cytoplasmic textures of stained cells. Lymphoma cells, splenocytes and peripheral blood lymphocytes (PBL) were stained with Papanicolaou stain and scanned on a Leitz MPVII microscope photometer operated on line to a PDP 11/45 computer. Two populations of lymphoid cell were detected in MD lesions using CAMA. One of these populations had features which were different from lymphoid cells from uninfected chickens and were termed "altered" cells. The other population had features which were similar to uninfected lymphocytes. Features were identified and selected that would classify cells as "altered" or "unaltered" and used to derive a decision rule to classify cells from other lymphoid populations. The decision rule classified up to 75% of cells from different lymphomas as "altered" cells. CAMA results were compared with clinical data and immunological data to quantitate T lymphocytes, B lymphocytes and MATSA (Marek's Disease Tumor Specific Antigen) positive cells. Results showed that there was a high correlation (r = 0.79) between the percentage of "altered" cells detected using CAMA and the percentage of MATSA positive cells. The majority of splenocytes and the PBL from the MDV group were classified as "unaltered".

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